Without seeing your 60' times, I would hazard to guess you spun a lot more on the launch of the 15.5s slip. The speeds are similar because once you got traction, the track wasn't much shorter than the time you got traction right away. So you sped up to almost the same speed.

For the record, your ET (Elapsed Time) never counts RT (Reaction Time). RT only matters in head-to-racing, not when seeing how fast your car is (ET).